Carbon monoxide (CO), a diatomic, colorless gas, has been shown to cause vasodilation, alleviate inflammatory responses, and provide graft survival during organ transplant. Endogenous CO is mainly produced by hemeoxygenases known as HO-1, HO-2, and HO-3 by catabolizing free heme. HO-2 is constitutively expressed in all cell types and may serve as a heme sensor as well as a regulatory factor for heme-responsive genes. An inducible isoform, HO-1, is affected by the external stimuli. Based on this knowledge, HO-1 knockouts mice are used in studying the beneficial effects of CO. Studies have shown that inhalation of CO rescued the HO-1 deficient mice from ischemic lung injury. HO-1 expression or its product CO demonstrated inhibition of vasoconstriction through the pathway independent of nitric oxide.
The anti-inflammatory effect of CO has been reported before by studying the endotoxin induced inflammatory responses. Lipopolysaccharides (LPS) from gram-negative bacterial cell wall were used as an inducer of inflammation in the HO-1 knockout animal models. Administering CO at low concentrations inhibited the LPS induced production of pro-inflammatory cytokines and tumor necrosis factor-α (TNF-α) while increasing the production of anti-inflammatory cytokine interleukin 10.
The CO anti-inflammatory effect can be seen not only in the animal models but also in cell cultures in vitro. Murine macrophages have been used in various studies to show the effects of CO releasing molecules (CORMs). Specifically, RAW264.7 cells are used for producing inflammatory cytokines by introducing lipopolysaccharides (LPS) from E. coli. The anti-inflammatory effect of CO produced from CORMs can be detected by observing the reduced amount of inflammatory cytokines.
Hemeoxygenases are potential targets for drugs in alleviating various disorders. In cases where reduced heme catabolism is desired such as neonatal hyperbilirubinemia and anemias, hemeoxygenases can be inhibited to prevent the release of iron and bilirubin. (U.S. Pat. Nos. 4,657,902, 4,699,903, 5,888,982) Another practice is to produce CO endogenously in treatment desired for the effects of CO. Hemeoxygenase inducers such as prostaglandins, vitamin B12, hemin derivatives, and compounds that decrease nitric oxide synthesis described in U.S. Pat. Nos. 6,066,333 and 5,891,689 may increase HO activity by inducing HO-1 expression. The problem with inducing HO-1 activity for the benefit of generating CO is that it also produces the two potentially toxic byproducts, bilirubin and iron.
The effect of CO can be obtained alternatively by administering CO gas as described in U.S. Pat. No. 5,664,563 or applying locally to organs before transplantation. Although the beneficial effects can be seen in much lower CO concentrations than that used in human pulmonary function tests, the CO gas inhalation as a therapy is not desirable since the danger may outweigh the benefits if it is not conducted under controlled conditions.
Carbon monoxide is the most commonly encountered environmental poison. Administration of carbon monoxide by inhalation is thus not practical for clinical applications, as it requires special delivery devices such as ventilators, face masks, tents, or portable inhalers. Moreover, carbon monoxide delivery to therapeutic targets by inhalation is inefficient, because it involves transport of carbon monoxide by hemoglobin. Hemoglobin binds carbon monoxide reversibly, but with very high affinity. Therefore, the doses required to deliver carbon monoxide to therapeutic targets in diseased tissues are likely to be associated with adverse effects.
Carbon monoxide releasing molecules (CORMs), however, is a potential therapeutic alternative that can deliver carbon monoxide directly to therapeutic targets without the formation of intermediate CO-hemoglobin complexes (see, e.g., Johnson et al., Angew Chem Int Ed Engl (2003) 42:3722-3729). These molecules can release CO inside the body system or the cells to bypass the inhalation process. The advantages of carbon monoxide delivery by CORMs over carbon monoxide delivery by inhalation are generally recognized. A few examples of CORMs include the molecules discussed in U.S. Pat. Nos. 9,163,044; 8,389,572; 8,236,339; 7,678,390; and 7,045,140.
However, CORMs need to be able to deliver carbon monoxide selectively to diseased tissues. The identification of CORMs that are best suited for the treatment of a particular disease remains a major challenge of CORM development. Thus, there continues to remain a need for CORMs which, upon administration in vivo, selectively target a particular disease or organ with therapeutic benefit.